Error correction encoding/decoding method and apparatus therefor

ABSTRACT

The invention provides greater error correction capability in an error correction encoding/decoding system of a digital mobile telephone or so forth. In a viterbi decoding system at a receiver side, a branch metric is calculated and stored without selecting a path with respect to a plurality of branches corresponding to a parameter having a correlation between frames. Also, by utilizing a separately stored parameter value in a previous frame, a transition probability of the parameter value in the current frame is obtained, a metric value of a path is converted and added to an accumulated path metric value up to the previous branch, a comparison of the metric values at various states is performed to select the path having higher likelihood to perform decoding by maximum likelihood decoding.

This application is a continuation of application Ser. No. 07/928,402,filed Aug. 12, 1992 (abandoned).

CROSS-REFERENCE TO RELATED APPLICATION

This application relates to U.S. Ser. No. 07/929,833 filed Aug. 14, 1992and entitled "Error Correction Encoding/Decoding Apparatus" being filedby Kazuyuki Miya and Osamu Kato, and assigned to the present assignee,based on Japanese Application No. 3-212285 filed on Aug. 23, 1991.

BACKGROUND OF THE INVENTION

The present invention relates to an error correction encoding/decodingmethod and an apparatus therefor to be employed in radio transmissionsystems, such as digital mobile telephones, portable telephones and soforth.

In the conventional digital mobile telephone systems or portabletelephone systems, a strong error correction encoding is employed inorder to maintain a predetermined level of quality even under poortransmission conditions (e.g. high channel error ratio). One of suchstrong error correction codes is a convolutional code. As a decodingmethod for the convolutional code, there is viterbi decoding employing atrellis diagram. As disclosed in "Points of Error Correction EncodingTechnology", Japan Industrial Technology Center Co., Ltd., published onMar. 20, 1986, pages 45˜48, or Japanese Unexamined Patent Publication(Kokai) No. 2-215232, viterbi decoding is a decoding method, in whichone of a plurality of known code streams, having the closest code metricto the received code stream, is selected as a maximum likelihood path,and decoded output data corresponding to the selected path is obtained.

FIG. 1 is a schematic block diagram of an error correctionencoding/decoding apparatus employing the conventional viterbi decodingmethod. For reception data a metric is calculated in a branch metriccalculation circuit 21 with respect to each branch (branch line from acertain state to the next state in the trellis diagram) corresponding toeach information bit. Then, the branch metric derived by the branchmetric calculation circuit 21 is added to a metric of an accumulatedpath (a sequence of the branches corresponding to the information signalstream) up to the immediately preceding branch stored in a path metricstorage circuit 23 by an ACS (Add-Compare-Select) circuit 22. Afterthat, the metrics in each state are compared, and then the maximumlikelihood path is selected and stored in a path memory 24 as a pathselect signal. Also, the value of a path metric is updated and stored inthe path metric storage circuit 23. Furthermore, the path metric valueis decoded by maximum likelihood decoding by a maximum likelihooddecoding circuit 25 and then output as decoded data.

Even with such conventional error correction decoding apparatus as setforth above, bit errors caused in the transmission line can be correctedto a certain precision level by utilizing a redundancy added inconvolutional encoding.

However, the conventional error correction encoding/decoding apparatusas set forth above is adapted to only use the redundancy added in theconvolutional encoding without using the redundancy of the informationsource even in the case that the parameter having correlation betweenconsecutive frames is included as a part of information bits to betransmitted so as to have a certain transition probability to define avalue in a current frame relative to the value in the previous frame.Therefore, under poor transmission conditions, there is high possibilitythat correct error correction cannot be performed.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide an error correctionencoding/decoding method and a apparatus therefor, which can performstronger error correction.

In order to accomplish the above-mentioned object, the present inventionperforms metric calculation at a receiver side with a transitionprobability of the parameter in addition to a redundancy added upon theconvolutional encoding, during viterbi decoding.

Also, according to the present invention, a viterbi decoding systemincludes storage means for storing transition probability of theparameter which has correlation between consecutive frames in a form ofa table, and calculation means for calculating a metric with respect toa sequence of a plurality of branches corresponding to the parameter, inwhich the metric value is converted with a value derived from the tableusing a parameter value of the previous frame.

Therefore, according to the present invention, a stronger and morereliable error correction can be realized by utilizing the redundancy ofthe transition probability of parameter of the information source per sein addition to the redundancy obtained through the convolutionalencoding.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a viterbi decoding system in theconventional error correction encoding/decoding apparatus;

FIG. 2 is a block diagram of a viterbi decoding system in one embodimentof an error correction encoding/decoding apparatus according to thepresent invention;

FIG. 3 is an illustration showing an information data structure in theshown apparatus;

FIG. 4 is a block diagram showing the major part of a encoding system inthe shown apparatus;

FIG. 5 is an illustration showing one example of a table to be employedin the shown embodiment; and

FIG. 6 is a block diagram showing another embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is a schematic block diagram of one embodiment of a viterbidecoding system according to the present invention. In FIG. 2, 2 denotesan ACS (Add-Compare-Select) circuit, 3 denotes a path metric storagecircuit, and 4 denotes a path memory (path select signal storagecircuit), 5 denotes a branch metric storage circuit, 6 denotes a metricconversion circuit, 7 denotes a parameter transition probability storagecircuit and 8 is a previous frame parameter storage circuit.

On the other hand, FIG. 3 shows an information data structure consistedof n bits per one frame. In FIG. 3, 17 denotes an m bits (0≦m≦n)parameter defining correlation between frames in the n bits informationdata. This parameter has certain transition probability defining thevalve in the current frame relative to the value in the previous frame.Having the transition probability, means that the m bits of the previousframe and the m bits of the current frame are not independentlygenerated information, having an independent relationship to each otherand thus the variable of probability of transition to 2^(m) ways ofvalues possibly taken in the next frame relative to the value in theprevious frame, and that, namely, the information source is a Markovinformation source.

At the transmitter side, as shown in FIG. 4, voice data is encoded by aencoding circuit 13 by a CELP (Code Excited Linear Prediction) or soforth. Next, in a data rearrangement circuit 14, the parameter 17 isstored in a frame together with the encoded data as set forth above toform the information data. Then, the information data thus formed isencoded by convolutional encoding in a convolutional encoding circuit15. Then, the encoded data is transmitted as transmission data. Here,parameters 18 and 19, having no correlation between the frames otherthan the parameter 17, are assumed as a bits and b bits respectively(note, a+b+m=n). At a receiver side, the decoded data is obtained by theviterbi decoding circuit having the construction illustrated in FIG. 2.

Next, operation of the above-mentioned embodiment will be discussed withreference to FIGS. 2 and 3. At first, in the parameter transitionprobability storage circuit 7, the transition probability of theparameter value of the current frame relative to the parameter value inthe previous frame is preliminarily set in the form of a table. Inpractice, assuming that the value in the previous frame is i, itrepresents the matrix of the probability p(j, i) to have the value j inthe current frame. It should be noted that, in the case set forth above,0≦i, and j≦2^(m) -1. FIG. 5 shows an example for the case where m=3bits. On the other hand, the parameter value of the previous frame isconstantly stored in the previous frame parameter storage means 8. Itshould be noted that since the shown system performs decoding processingwith respect to each frame, the parameter value stored in the previousparameter storage circuit 8 is updated at every timing when the decodeddata is obtained with respect to each frame. On the other hand, in theviterbi decoding, branch metrics are calculated for the branchescorresponding to the information bits a and b of the parameters 18 and19 by the branch metric calculation circuit 1 in the similar manner tothe conventional system. Then, in the ACS (Add-Compare-Select) circuit2, the calculated branch metric is added to an accumulated path metricvalue up to the previous branch, which is stored in the path metricstorage circuit 3 for comparison of the metric values at various statesto select the path having the higher likelihood. The selected path isstored in the path memory 4 as a path select signal. Also, the value ofthe path metric is updated and stored in the path metric storage circuit3.

On the other hand, in calculation of metric with respect to a sequenceof a plurality of branches (m branches) corresponding to the parameter17, instead of selecting the path for each branch as in theabove-mentioned branches, without selecting the path for m branches, thebranch metric value is, at first, calculated by the branch metriccalculation circuit 1 and the calculated value is stored in the branchmetric storage circuit 5. Then, utilizing the parameter value in theprevious frame, which is stored in the previous frame parameter storagecircuit 8, the transition probability of the parameter value of thecurrent frame is obtained from the transition probability table storedin the parameter transition probability storage circuit 7. Thereafter,the metric values (i.e. partial path metrics) of 2^(m) paths of thelength m gathered in various states are converted by the metricconversion circuit 6. Here, assuming that the data to be handled is theencoded data by the encoder having an encoding ratio of R=1/2, themetric before conversion is referred to as CR (Channel Redundancy)metric and the metric after conversion is referred to as SCR (Source &Channel Redundancy) metric, conversion is performed with the followingconversion equation in the metric conversion circuit 6.

    SCR metric=CR metric×log.sub.2 {1/p (j, i)}/m

here, m is the number of bits of the parameter containing the transitionprobability of p(j, i).

Subsequently, in the ACS circuit 2, the accumulated path metric value upto the previous branch, which is stored in the path metric storagecircuit 3, is added to the metric value (SCR metric value) for the mbranches after conversion. Comparison of the summed metric values isthen performed in various states to select the path having the highlikelihood and stored in the path memory 4 as the path select signal.Then, the decoding is performed by a maximum likelihood decoding circuit9 to output the decoded data.

As set forth above, by the foregoing embodiment, in the case oftransmitting the information data including the parameter 17 havingcorrelation between the frames, the parameters 17 is encoded as acontinuous data series upon the convolutional encoding. On the otherhand, in the viterbi decoding system at the receiver side, there areprovided: the branch metric storage circuit 5, the previous frameparameter storage circuit 8 for storing the value of the parameter 17having the correlation between the frames, in the previous frame, theparameter transition probability storage circuit 7 storing thetransition probability of the parameters 17 and the metric conversioncircuit 6 performing conversion of the metric. With respect to aplurality of branches corresponding to the parameter 17 having thecorrelation between the frames, the metric calculation is performed withcombining the redundancy of the parameter transition probability of theinformation source in addition to the redundancy obtained through theconvolutional encoding, and selection of the path is performed basedthereon. Therefore, the stronger and highly reliable error codecorrection can be realized.

In addition, another embodiment is illustrated in FIG. 6, in which theparameter transition probability storage circuit 7 does not fix thevalues in the table, and updates the values of the table at apredetermined condition, for instance, when a difference between theparameter transition probability stored in the table and the actualparameter transition probability is maintained greater than or equal toa predetermined value over a given number of frames. It should be notedthat the following discussion will be given for the components added tothe construction of FIG. 4.

Referring to FIG. 6, a parameter transition probability calculationcircuit 10 calculates an actual parameter transition probability on thebasis of the parameter value stored in the previous frame parameterstorage circuit 8 and the parameter values decoded in further previouscycles. Next, the transition probability comparing circuit 11 comparesthe transition probability stored in the table, in the parametertransition probability storage circuit 7, with the actual parametertransition probability obtained by the parameter transition probabilitycalculation circuit 10. Here, comparison is performed to determinewhether the difference of both probabilities exceeds a specificreference value. At this time, when the difference exceeds the specificreference value, a counter included in the system is counted up andotherwise the counter is reset. The process is performed for everyframe. When the difference of both probabilities exceeds the specificreference value over a given number of frames, a new transitionprobability is calculated by a new transition probability calculatingcircuit 12 to update the transition probability table of the parametertransition probability storage circuit 7, based on the comparison in thetransition probability comparing circuit 11.

As set forth above, by the shown embodiment, by updating the originaltransition probability table in the decoding side based on the resultsof decoding during the actual data transmission, the erroneouscorrection probability can be reduced as a result of proper decodingeven when the transition probability of the parameter havingcorrelation, such as voice, is differentiated with respect to eachindividual.

What is claimed is:
 1. In a transmission system for transmittinginformation in frames, each frame having a plurality of parameters, anerror correction encoding/decoding method comprising the steps of:(a) ata transmitter side of said transmission system, (i) placing acorrelation parameter indicative of a correlation between frames in afixed part of each frame, (ii) performing a convolutional encoding ofthe frames, and (iii) transmitting the encoded frames as errorcorrection code; (b) at a receiver side of said transmission system,performing Viterbi decoding, including (i) performing calculation of ametric indicating a transitional probability of each branch of a trellisdiagram using a first redundancy derived from convolutional encoding forreceived parameters not having the correlation between frames, (ii)performing calculation of the metric using the first redundancy derivedfrom the convolutional encoding and a second redundancy derived fromcorrelation between frames for received parameters having thecorrelation between frames.
 2. An error correction encoding/decodingapparatus for transmitting information in frames, each frame having aplurality of parameters, said apparatus comprising:an encodercomprising: encoding means for (i) placing a correlation parameterindicative of a correlation between frames in a fixed part of eachframe, (ii) performing a convolutional encoding of the frames, and (iii)transmitting the encoded frames as error correction code; and a Viterbidecoder comprising: calculation means for (i) calculating a metricindicating a transitional probability of each branch of a trellisdiagram using a first redundancy derived from convolutional encoding forreceived parameters not having the correlation between frames, and (ii)calculating the metric using the first redundancy derived from theconvolutional encoding and a second redundancy derived from correlationbetween frames for received parameters having the correlation betweenframes.
 3. An error correction encoding/decoding apparatus fortransmitting information in frames, each frame having a plurality ofparameters, said apparatus comprising:an encoder comprising: encodingmeans for (i) placing a correlation parameter including a plurality ofbits indicative of a correlation between frames in a fixed part of eachframe, (ii) performing a convolutional encoding of the frames, and (iii)transmitting the encoded frames as error correction code; and a Viterbidecoder comprising: transition probability storage means for storingtransition probabilities of said correlation parameter with respect tothe correlation parameter value of a previous frame; previous frameparameter storage means for storing the correlation parameter value ofthe previous frame; branch metric storage means for storing a metricindicating a transitional probability of each branch of a trellisdiagram; path storage means for storing a selected path of the trellisdiagram; path metric storage means for storing a path metric indicativeof an accumulation of metrics of selected paths; and means for (a)determining whether a received bit belongs to said correlationparameter, and (b) if said received parameter bit does not belong tosaid correlation parameter, then (i) calculating a branch metric forsaid received bit, (ii) adding the branch metric to the accumulationstored in said path metric storage means, (iii) selecting a path havinga greater metric value, (iv) storing the selected path in said pathstorage means, (v) updating said path metric value in the path metricstorage means, and (c) if said received bit belongs to said correlationparameter, then (i) calculating the branch metric for said received bit,(ii) storing the branch metric into said branch metric storage means,(iii) repeating the operations of (c)(i) and (c)(ii) for said pluralityof bits belonging to said correlation parameter, (iv) obtaining partialpath metrics corresponding to said plurality of bits belonging to saidcorrelation parameter from the branch metrics stored in said branchmetric storage means, (v) getting a transition probability of thecorrelation parameter of a present frame from said transitionprobability storage means using the correlation parameter of theprevious frame, stored in said parameter storage means, (vi) convertingthe path metrics using the transition probability stored in saidtransition probability storage means, (vii) adding the converted metricto the accumulation stored in said path metric storage means, (viii)selecting a path having a greater metric value, (ix) storing theselected path in said path storage means, and (x) updating the pathmetric value of said path metric storage means.
 4. An error correctionencoding/decoding apparatus as set forth in claim 3, wherein saidViterbi decoder further comprises:transition probability calculationmeans for calculating a transition probability of the correlationparameter from decoded frames; transition probability comparing meansfor comparing the transition probability calculated by said transitionprobability calculation means with a transition probability stored insaid transition probability storage means; and new transitionprobability calculation means for updating said transition probabilitystored in said transition probability storage means using apredetermined standard and a result of said transition probabilitycomparing means.
 5. An error correction encoding/decoding apparatus asset forth in claim 4, wherein said new transition probabilitycalculation means updates the transition probability when a differencebetween the transition probability derived from the decoded frames andthe transition probability stored in said transition probability storagemeans is continually greater than or equal to a given value over apredetermined number of frames.
 6. An error correction encoding/decodingapparatus as set forth in claim 3, wherein said transition probabilitystorage means stores said transitional probabilities of said correlationparameters with respect to the correlation parameter value of a previousframe in table form.